The detection of contraband (e.g., explosives and drugs) in closed containers is of growing importance worldwide. World events have necessitated tighter screening requirements for the contents of containers placed on aircraft to detect the presence of explosives. In addition, to combat the illegal flow of contraband across national borders, the contents of containers, such as loaded trucks and vehicles must be inspected to check for the presence of narcotics. High energy x-ray inspection systems remain one of the only technologies capable of inspecting loaded cargo containers and vehicles.
There are many detectors used for the detection of x-rays, gamma rays, and charged particles. These detectors typically have unique properties which make them suitable for particular applications. These applications include scintillating screens used in conventional x-ray machines and a variety of single crystal materials used in many nuclear physics applications.
U.S. Pat. No. 4,503,332 discloses a detector suitable for use as a transmission detector in a flying spot x-ray system. This transmission detector uses a thin scintillating screen (or two screens mounted back-to-back) oriented at a grazing angle to the incoming pencil beam of x-rays. The grazing angle allows a long path for the x-ray beam, while the light photons that are produced by the x-ray beam can exit through the thin dimension of the screen. The reason for this geometry is that the scintillating screens have a short attenuation length (of the order of 0.3 mm) for the light photons which are detected by a photo-multiplier tube to form the x-ray image.
This prior art transmission detector works sufficiently well for an x-ray beam with a peak energy in the range 50 to 200 kV. Above this energy this detector design is not efficient for several reasons. First, since the maximum thickness of the scintillating screen is approximately 0.5 mm, the grazing angle must be made as small as possible in order to increase the path length of the x-ray beam in the screen(s). For example, a grazing angle of 3.degree. (on back-toback 0.5 mm screens) results in an x-ray path length of about (0.1/tan 30.degree.) 2 cm in the scintillating screen. This path length is sufficient to detect about 40% of the x-rays at an energy of 500 kV. Second, the x-ray pencil beam typically has a cross section of about 3 cm .times.3 cm as it enters the transmission detector. Thus, the depth of the detector must be at least (3/tan 3.degree.) 60 cm, or approximately two feet for the transmission detector. Significantly, a transmission detector of this depth requires a significant amount of heavy shielding (e.g., lead), which increases the cost and the size of the system.
Prior art scatter detectors are also inefficient at high energy. For example, conventional scatter detectors use a thin scintillating screen mounted on the front face of the detector. Alternatively, the thin scintillating screen may be mounted on another of the other five faces of a rectangular detector embodiment. In either of these embodiments the other five faces of the rectangular detector are covered with a light reflecting material. The scatter detector must be of an area large enough to subtend a large solid angle at each point along the line where the primary pencil beam of x-rays interacts to produce a cone of scattered x-rays. As known, the maximum thickness of the scintillating screen is about 0.5 mm. Above this thickness the visible light produced by the x-rays can not escape from the scintillating screen. This detector design has proven acceptable for systems that employ an x-ray beam with a peak energy in the range of 50-150 kV. For an energy of the x-ray photon of 100 kV, the efficiency of the screen to stop and detect this photon is approximately 40%. However, for an energy of the x-ray photon of about 200 kV, the efficiency of the screen to stop and detect this photon is only 8%. The efficiency decreases further at higher energies of the x-ray photons.
Therefore, there is a need for an improved x-ray detector for a high energy x-ray inspection system.